The subject invention relates generally to thermal ink jet printers, and is directed more particularly to a technique for reducing drive energy in thermal ink jet printheads while maintaining consistently high print quality.
An ink jet printer forms a printed image by printing a pattern of individual dots at particular locations of an array defined for the printing medium. The locations are conveniently visualized as being small dots in a rectilin-ear array. The locations are sometimes "dot locations", "dot positions", or "pixels". Thus, the printing operation can be viewed as the filling of a pattern of dot locations with dots of ink.
Ink jet printers print dots by ejecting very small drops of ink onto the print medium, and typically include a movable carriage that supports one or more printheads each having ink ejecting nozzles. The carriage traverses over the surface of the print medium, and the nozzles are controlled to eject drops of ink at appropriate times pursuant to command of a microcomputer or other controller, wherein the timing of the application of the ink drops is intended to correspond to the pattern of pixels of the image being printed.
Thermal ink jet printheads commonly comprise an array of precision formed nozzles, each of which is in communication with an associated ink containing chamber that receives ink from a reservoir. Each chamber includes a thermal resistor which is located opposite the nozzle so that ink can collect between the thermal resistor and the nozzle. The thermal resistor is selectively heated by voltage pulses to drive ink drops through the associated nozzle opening in the orifice plate. Pursuant to each pulse, the thermal resistor is rapidly heated, which causes the ink directly adjacent the thermal resistor to vaporize and form a bubble. As the vapor bubble grows, momentum is transferred to the ink to be propelled through the nozzle and onto the print media.
For gray scale printing, wherein the darkness of each printed dot is varied, it is known to vary the volume of ink in each drop that produces a printed dot. For example, commonly assigned U.S. Pat. No. 4,503,444 for "METHOD AND APPARATUS FOR GENERATING A GRAY SCALE WITH A HIGH SPEED THERMAL INK JET PRINTER," incorporated herein by reference, discloses a thermal ink jet printer wherein each drop is formed pursuant to a pulse group applied to a resistor which causes emission of a packet of droplets that merge in flight to form a single drop.
A consideration with the operation of thermal ink jet printheads with drop forming pulse groups is increased printhead operating temperatures due to multiple firings for each pixel. This consideration becomes more notable with small drop volume thermal ink jet devices which require relatively higher input energy per unit flow of ink, and thus develop higher operating temperatures as a result of the increase in average power.
High operating temperatures are known to cause degradation in print quality due to induced variability in printhead performance parameters such as drop volume, spray, and trajectory. Moreover, when the operating temperature of a thermal ink jet printhead exceeds a critical temperature, it becomes inoperative. Also, the operating lifetime of a thermal ink jet printhead can be reduced as a result of excessive heat build up.
A common technique for reducing heat build up is to operate at lower resistor firing frequencies, which delivers lower average power to the printhead. However, reducing the maximum resistor firing frequency also reduces printing speed and throughput.